Radio receiver circuit



Feb. 29., 1944. D E NQBLE 2,343,115

RADIO RECEIVER CIRCUIT Filed April 5, 1941. 2 Sheets-Sheet 1 Feb. 29, 1944.A D1 E NOBLE I 2,343,115

RADIO RECEIVER CIRCUIT Filed April 5, 1941 2 Sheets-Sheet 2 Patented F eb. 29, 1944 UNITED STATES PATENT OFFICE RADIO RECEIVER CIRCUIT Daniel E. Noble, Chicago, Ill., assignor to Galvin Manufacturing Corporation, Chicago, lll., a

corporation of Illinois Application April 5, 1941, Serial No. 386.989

14 claims. (cl. 25o-zo) My invention relates in general to a radio receiver system having means for producing a voltage which is preferably employed to operate a muting circuit in the system but which may also be used for other purposes within the capabilities of the magnitude of such voltage. The invention relates more in particular to a receiver circuit, and a muting or squelch system therefor, particularly adapted for use in communication equipment for emergency and military service, to substantially maintain silence at the loud speaker or similar reproducer of a receiver in such equipment while the receiver is retained in continuously operating condition ready for immediate reception of a signal at any time it is transmitted to the receiver.

As is well known, receivers in communication equipment for emergency and military 4service particularly, are normally maintained tuned to a single frequency and in an operating condition, so that the receiver will always pick up and reproduce at any time signals of the desired frequency which might be put on the air continuously or intermittently. With the receiver maintained in an operating condition, in the absence of a carrier the receiver will reproduce noise unless some system is employed to mute the audio circuit or other appropriate portion of the receiver system. This noise is extremely annoying at best, and becomes unbearable in many in-l stances. When such muting means, or squelch system, as itis generally called, is applied to a receiver, the reproducing means and normally the audio system is rendered inoperative until it is automatically rendered operative by the reception of a carrier wave at the input of the ref ceiver. Thus, the muting or squelch system permits the operation of the receiver in a normal condition ready for the reception of a desired signal, but without reproducing undesirable noise in the meantime.

For the purpose of this disclosure, the process of rendering the radio receiver output quiet will be described as muting or closing the squelch, while the process of making the radio receiver operative for the purpose of reproducing desired communications, will be described as opening the squelch.

Muting or squelch systems for the above noted purpose are old in both amplitude modulated receivers, and in frequency modulated receivers. However, in 'the prior art muting or squelch systems, noises such as electrical interference from ignition and other man-made sources, will open the squelch system when it is adjusted for the Il most sensitive and hence desirable operation even though no carrier is present. This undesirable result is due to the fact that the prior systems have as their controlling vfunction, an average increase in the average signal and the noise passing through the radio receiver. Thisv undesirable result is particularly noticeable in areas of high noise level such as areas of heavy trailic, or where there are other sources of electrical interference. It is particularly disturbing to an operator and interferes with the emcient use of the receiver apparatus. In these prior devices when the squelch system is adjusted so that man-made noises will not open the squelch system and hence render the audio-system operative, than the receiver is not sensitive to the reception of carrier waves of low level and often-times messages are inaudible to the listener. 'I'he seriousness or this condition in the use of communication equipment for emergency and military service is obvious.

As can be understood from the above discussion, this diiliculty can, and did arise in mobile receivers as well as in iixed receivers, where the noise level in the surrounding locality might vary from time t0 time. In these prior systems as the noise level decreased the receiver was adjusted for more sensitive signal operation. This would permit noise, if it increased in intensity, to openy the squelch in the absence of a carrier, and be reproduced. Another adjustment of the squelch system in the receiver would then become necessary, and this continued changing was not only troublesome and time consuming, but interfered with eillcient operation of the equipment.

It is an object oi my invention to provide an improved muting or squelch system for radio apparatus.

A further object of my invention is to provide a frequency modulated radio receiver having improved muting or squelch means therein.

It is also an object of my invention to provide an amplitude modulated radio I receiver having improved muting or squelch means therein.

Another object of my invention is to provide a muting or squelch system for a radio receiver so that the receiver may be maintained at all desired times in the most sensitive operating condition and ready for signal reception, and yet remain silent in the absence of a signal in areas of noise due to electrical disturbances, which noise may vary widely in intensity and frequency.

A still further object of my invention is to provide a squelch system which willl open on a carrier of low signall level as well as high 8181181 level, so that reproduction of all desired sisnals is possible, and yet the receiver will be muted andthesquelchsystcmrctainedclosedintheabsence of a carrier at all levels of ignition noises or other Vman-made noises.

' modification of Fig. 8, which can be substituted in the complete It is also an object of my invention to provide 1 a sensitive squelch system remaining closed at high noise levels and opening at low carrier levels, and to accomplish these desirable operating conditions in a simple and effective circuit.

A feature of my invention is the provision of aradioreceiversystemwhich produces a source of voltage therein that is enective to operate anv electronic muting or squelch system, to operate a. relay for Vaccomplishing muting, or to operate other electronic or electrical devices for other muting device such as a squelch or audio-muting system, or to control a mechanical relay connected into the loud speaker circuit Volf the receiver circuit, or it may be used for other special pur-v posesinsuchreceivercircuit. 'Y

Another feature of my invention is the provision of a squelch or muting system which is unusually sensitive to a desired carrier or signal,

and yet at the same time is substantially entirely insensitive to undesired signals or noise at high as well as low power levels. Y

A still further feature of my invention is the provision of a. method for squelch control which is equally effective for either a frequency o'r an amplitude modulated radio receiver under all types of operating conditions.

Other objects, advantages, and featuresofmy invention will be apparent from the following description taken with the drawings, in which:

Fig. 1 is a schematic illustration of a frequency modulation receiver circuit;

Fig. 2 is a modication of the selective circuit portion of the complete circuit of Fig. 1, and is adapted to be substituted for the corresponding portion in Fig. 1;

Fig. 3 is a circuit-diagram of another modification of the selective circuit portion of the receiver circuit of Fig. 1 and may likewise be substituted for the corresponding portion in the circuit of Fig. 1;

Fig. 4 illustrates diagrammatically a band pass circuit for selecting a band of noise frequencies below the communication frequencies, and this circuit can 'be substituted for the corresponding frequency band selecting portion of Fig. l;

Fig. 5 is a simplified and abbreviated block diagram introduced purely for purposes of clarity to show a portion of the squelch control system in Fig. 1 in a different manner from that in which it is illustrated in Fig. 1;

Fig. 6 is a circuit diagram after the fashion for the corresponding portion system of 8.

Referring now to' Fig. 1, a. frequency modulated radio receiver systan is illustrated including an antenna A. aspeaker 8, .and a receiver circuit for translating the frequency modulated waves received at the antenna A into reproductions at the speaker S, or a similar device, The receiver circuit includes an R. Il'. amplifier Il, a converter and oscillator Il, an I. llamplifler I2, a second converter ll,'and an I. P. amplifier Il, all connectedincascadeandinturnconnectedtoan inductance coil It. Just described is conventional and no detailed diagram has been illustrated. The remainder of the circuit as illustrated includes a limiter tube l1, with a grid Il, and a balanced frequency detector, or discriminator portion including the tube I9. The remainder of the circuit also includes an audio-tube or noise amplifier 20 in a selective system, as will be-hereinafter described, a rectifier tube 2l, and an audio amplier 22. The amplifier 22 is preceded in the circuit by a squelch tube 23 with a grid 24 which tube controls the operation of the audio-amplifier and hence determines reproduction at the speaker S, or similar device.

As to the balanced frequency detector, or discriminator portion of the receiver circuit, this includes a primary circuit condenser a, and coil b, condenser c, secondary coil d, and condenser e. The primary circuit is coupled inductively to the secondary circuit through the mutual inductance between the coils b and d, and they are coupled capacitively through condenser c. The terminals of the secondary coil circuit connect to the plates of the diode, or frequency detector tube I9, and the rectified current flowing through the diode passes through resistors 3l and 30'. The action of this circuit portion is to translate a frequency modulated wave into a hybrid wave which includes both frequency modulated and amplitude modulated components. After that, the amplitude modulated audio-signal component is recovered in the well known manner.

When a pure amplitude modulated wave appears,at the primary circuit including condenser a and coil b, equal and opponte voltages will appear at the ,secondary terminals. The .rectined currents ythrough resistors 3l and 30' will vary at the modulating signal frequency in phase oppomtion and in such a manner that the potential from point I to point y in the circuit portion will remain zero in a perfectly balanced system. Hence the audio response at the output will be zero for a pure amplitude modulated wave input.

of the diagram of Fig. 5, showing the complete v Fig. 9 illustrates a fragmentary portion of a In other words, the balanced frequency detector will detect variations in frequency of ythe wave, but will not detect or recover amplitude variations of an amplitude modulated wave.

For abetter understanding of the complete system of Fig. l, particularly as to the operation of the squelch or muting portion of the system, one possible means for operation of the squelch portion will be explained preliminary to the description of the complete system. A noise voltage received at the input to the radio receiver or developed in therinitial stages of the receiver system is amplified and introduced at the grid ll of the limiter tube I1 which produces a rectified current flowing through resistor 28. 'I'his will.

develop a potential across the resistor 26 varying in magnitude with the variation of the amplitude of the noise voltage arriving at the grid I8. The

The portionv of the circuit potential across resistor -28 is negative with respect to ground and will pass through the circuit including the resistors 21, 28 and 29. to

` the grid 24 of the squelch tube 23. The negative gain of the amplier 22 will be changed so that y such amplifier will not function, and hence the speaker S will fall to operate. The amplifier 22 can be controlled in this manner by applying 'an abnormally high negative potential at the grid of one or more tubes in the amplier 22, or by greatly reducing the potential at the plate of such tube or tubes.

.From the above explanation, it is clear that with this type of operation, noise voltages will open the squelch if they attain a necessary voltage level, and voltages of the same magnitude generated by a carrier in the receiver system will operate the squelch in the same manner.

Noise voltages ofalternating current character, different, of course. from the voltages produced by the rectified current or integrated D. C. in resistor 26, may be prevented from reaching the tube 23 by the use of a suitable filter or time delay network including capacitances 32 and 33, resistors 21 and 29, and capacitance 34. These elements will serve as a de-coupling lter to prevent audio-frequencies of an A. C. character from passing to the grid 24. The rectiiied current producing a corresponding noise voltagev will pass to the grid 24 of the squelch tube 23 in the absence of a carrier in the receiver system when sui'llcient integrated voltage accumulates across resistor 28, and this voltage at the grid 24 will open the squelch. It is undesirable to open the squelch with noise, and hence reproduce noise at the speaker, and this dimculty becomes particularly serious, as can be understood, in an area where a great deal of noise abounds. Under such circumstances an operator will be forced to listen to noise continuously or intermittently reproduced at the speaker in the absence of a carrier in the system. This noise reproduction can be reduced or eliminated by varying the value of the resistor 26 so as to reduce the sensitivity of the response of the squelchvtube 23 to integrated noise voltages. When the sensitivity is reduced, however, to prevent the opening of the squelch by noise, the squelch is also less sensitive to carrier signals. Under such circumstances it is possible that a carrier signal of relatively low level, but a level which normally should be audible at the speaker, is not of suiiicient strength to open the squelch and permit the operation of the ampliiler 24, and the speaker.

In a receiver system such as illustrated in Fig.

1. however, this dliculty with reference to noise, and lack of sensitivity to carrier signals, can be overcome by utilizing a potential developed across the Iresistor 28 to oppose the potential developed across the resistor 26. To simplify the explanation of the development of this potential across the resistor 28, an FMl circuit ls'illustrated diagrammatically in Fig. 5, in which merely the principal elements employed to accomplish this operation are shown diagrammatically. The operation will be more clear by a reference to both Figs. 1 and 5. The block diagram indicated by the reference character N in Fig. 5 corresponds generally to the ampliiier lvoltage thereacross.

to a frequency modulated receiver system, a

tube 29 whose grid 38 is coupled to the output of the frequency detector I9 and the selective system or filter as will be described.

In the absence of a carrier at the input to the receiver, vnoise voltage generally made up of thermal agitation potentials and shot noise introduced at the input to the receiver or developed'in the R. F. amplifer and converter stages, is amplied through the successive stages of the receiver and appears at the output of the detector I9 to,v be coupled to the grid 36 of the noise amplifier tube 29, through the condenser 31, and the resistor 38. The noise voltages are amplified in the tube 20 to increase the intensity of the voltage` at the plate 35 of such tube, y

and desired frequencies are selected in the tuned circuit consisting of condenser 4I, inductance 42, condenser 43, and lnductance 44. circuit is tuned to select a band of frequencies in the noise spectrum above the range of audiofrequencies used in the complete receiver systern for communication purposes. The circuit including the tube 29 and the selective system described acts as the combination noise amplifier and filter N as mentioned above. For instance, in one embodiment of the system described, voice communication is used with a maximum frequency for voice reproduction of 3,000 cycles. The noise band selected by the selective system described may be anywhere above 3,000 cycles, and this noise band should be selected in a manner such that the tuned circuit described will not pass the audio-frequencies or voice frequencies below 3,000 cycles. In practice, the tuned selective circuit has been tuned to 8,000 cycles, to 12,000 cycles, or 'to 20,000 cycles when the voicey reproduction is 3,000 cycles.

After amplifying and selecting the band of noise voltages, such noise voltage is rectied by rectifier tube 2|, and the rectified current flows in resistance, or resistor 28 to produce a voltage with a polarity as shown in' Figs., 1 and 5. With no carrier voltage at the input to the receiver, this rectied noise voltage appearing across resistance 28 will be large in magnitude, and maybe made to reach any desired value by means of suitable amplication. With ordinary amplification voltages ranging from 10 to 60 volts have been developed in-various embodiments of the invention. Where additional voltages have been required without the use of further ampliilcation in the circuit, the tube 2l, which can be a standard 6H6 tube, consisting of a double rectifier element construction can be connected into a conventional voltage doubling circuit so as to produce the increased potential across resistor 28, without votherwise changing the illustrated circuit. As illustrated, only one of the diode portions of the tube 2| is being utilized. The potential developed across resistor 28 as a. result of noise voltages in the receiver system acts on the squelch 23 to maintain it closed.

Means have been described above including a noise amplier for amplifying the noise voltages at the output of a detector system, and a selective circuit for selecting a band of noise voltages which are rectified by a rectier tube and and applied to a resistance to produce a When a carrier is applied limiter such as the limiter I1 acts to reduce the level of noise appearing at the output of n detector tube as the detector I9. Inasmuch as This the noise voltage appearing across the resistor 28 was developed in the receiver circuit and appeared as a noise voltage at the output of the detector I8, as previously described, the rectified. noise voltage just described as appearing across the resistor 28 will be reduced as a result of the application of a carrier voltage to the input of the FM receiver. In fact, in a properly designed receiver with a limiter, or with balanced detection, or both, a small carrier voltage applied at the input, as the antenna A, can be made to produce a large reduction in noise voltage within the receiver, and hence can likewise be made to produce a large reduction in the rectified noise voltage across the resistor 28. In one particular embodiment of the invention, a reduction of 20 decibels of noise voltage at the output of the receiver was accomplished, as a result of injecting a carrier voltage of 094 microvolt into the input of the receiver.

In an FM receiver. or in fact, an AM receiver, as will be described, noise reduction or silencing may be achieved in the presence of a carrier, when all R. F. and I. F. circuits are tuned to exact resonance with the carrier, and the balanced .discriminator or frequency detector circuit portion is tuned to precise balance with the carrier. Under such a condition the limiter may have little additional eiect in reducing the noise in the presence of a carrier. This action all results from the lack of response of such detector system to amplitude modulation so that the noise side bands produced by the beating of noises with the incoming carrier will be balanced out at the output of the detector system. However, in communication equipment of the type particularly described herein, precise tuning at least with present equipment cannot be maintained in a practical manner in the rugged hard usage to which it is put. However, the limiter will act to eliminate the amplitude variations over reasonably wide limits of detuning from the carrier frequency, and it therefore becomes a more practicable means for all operating conditions to accomplish noise reduction.

It is apparent, therefore, from the above description, that the present invention provides a system in which the noise voltage across the re sistance 28 may be made large in the absence o. a carrier input to the receiver and with this volt age applied to the grid 24 of the squelch tube 28 the squelch is retained closed. This voltage, how ever, may be reduced by the application of a small carrier voltage to the input to the receiver, and the squelch system could be so designed that this change from a high potential across resistor 28 to a low potential, in the presence of a carrier, will open the squelch and permit the operation of the amplifier 22 in the speaker S.

A more desirable, and in fact the preferred means for operating the squelch system of my invention in the complete receiver system of Fig. l, is to effect a differential action of two voltages to open and close the squelch. The complete system illustrated in Fig. 1 is illustrated in an abbreviated manner in Fig. 6 to show more clearly the more essential elements of the system. Fig. 6 is presented merely in the interest of clarity of understanding of the invention, the same as for Fig. 5. as previously explained.

The differential action involves voltages developed across the resistor 26, and also the resistor 28. The production of the voltage across the resistor 28 has been discussed in detail, and some mention was originally made of the production of a voltage across the resistor 26. Briefly, as to the latter, a carrier voltage, or noise voltage, appearing in the receiver circuit at the` inductance I6. and rectified at4 the grid I8 of the limiter tube l1, will produce a'voltage across the resistor 26 which is negative in respect to ground.

In the case of the application of a. carrier at the input to the receiver, there will be an increase in the voltage across the resistance 28 that will normally tend to open the squelch. Inasmuch as a noise voltage will normally always appear across the resistance 26, whether there is a carrier, or is not a carrier at the input of the receiver, this voltage at resistance 26, regardless of its nature, will tend to open the squelch unless it is kept at a low magnitude, or opposed by a voltage of opposite polarity.

In the present system, the voltage across the resistance 26 is opposed by a voltage across the resistance 28 of opposite polarity, as is shown in Fig. 1, but possibly more clearly in the abbreviated diagram of Fig. 6. In order to 'close the squelch 23 by opposing the voltage across the resistance 26 with the voltage across the resistance 28, the latter must be adjusted to a suitable value. This adjustment may be made by changing the amplification produced by the amplifier tube 28, and in the circuit of Fig. 1 this is accomplished by varying the magnitude of the resistor 46. The voltage across the resistor 28 may also be changed by varying the voltage of the noises introduced to the grid of the noise amplifier tube 28 from the detector tube I8. This adjustment is made until the diierential between the two voltages is such that the squelch just closes with a noise voltage across each resistor.

With the receiver system energized but the audio-amplifier 22 and loud speaker S muted by the closed squelch, as described, there will be silence until a carrier is introduced to the receiverthrough the antenna A. A carrier, or a modulated signal, will produce an increase in the negative voltage across the resistance 26 as previously described. Simultaneously, with the action of the frequency modulated system upon the introduction of a carrier, there will be a reduction in the noise voltage appearing at the output of the frequency detector I8 and this will be translated into a reduction in the voltage appearing across the resistance 28, as previously described. In other words, the carrier, or modulated signal, increases the negative potential tending to open the squelch 23 while at the same time it decreases the potential across the resistor 28 tending to keep the squelch closed. This differential action produces a very positive and sensitive control for the squelch 23, which in turn controls the output from the receiver.

The amplifier 28 input from the detector Il can be controlled by an adjustment of the elements of the intervening circuit,` or the amplification of the tube 28 may be varied by the variation of the resistor 46, so that in all the voltage appearing across the resistor 28 can be made -to be substantially greater' than that necessary to just maintain the squelch in a closed condition in the absence of a carrier. In this condition, noise voltages in the receiver system, although they may produce substantial noise voltages across resistor.` 26, tending to open the squelch, such volttages will always be opposed by a voltage across resistor 28 which is increased to a point that it will definitely prevent the opening of the squelch by opposing the voltage at the resistor 26. Inasmuch as a carrier in the receiver will immediateassaut 1y substantially reduce the voltage appearing d across the, resistor -26. however. the differential action will accomplish an unbalanced condition between the voltages across resistors 26 and 28, and will permit the predominant negative voltage from the carrier at theresistor 26 to open the squelch. The squelch system as described will operate in a very sensitive manner.

Of course, actually, with a definite decrease in the voltage appearing across the resistor 28 upon the introduction of acarrier, the voltage across the resistor 26 could remain at ,the same value and the differential action would weigh in favor of the voltage opening the squelch. By the same token, inasmuch as there is this reduction of voltage at the .resistor 28 upon the introduction of a carrier, it is not necessary under any circumstances for the voltage across the resistor 28, in the absence of a carrier, to overcome the opposing action of the voltage across resistor 28. There is a further action of this complete system which tends to make it insensitive to undesirable operation by noise voltages in thereceiver system. Although the application of additional noise from within the receiver itself, or manmade noise introduced to the input of the receiver, will tend to increase the voltage across resistor 26 and hence tend to open the squelch A123, this same noise voltage will pass through the receiver system to increase the noise output at -the detector I6, and increase the magnitude of the opposing voltage across resistor 28. Theredesired to be reproduced in the receiver system ments corresponding to those in the selective system of Fig. 1 are numbered the same as in Fig. 1. Fig. 2 illustrates a simple well known parallel resonant circuit including a condenser or capacitance 6I, and an inductance 62. The system is resonant above the audio-frequencies just as was the selective system of Fig.A l.

Fig. 3 illustrates diagrammatically a high pass vfilter which may also be used to select frequencies above the audio frequencies used for comv munication transmission in the complete receiver system. This circuit includes the conventional i inductances I3, 54, and l5, together with capac- The operation of this circuit is readily unselected band of noise voltages is, of course, rectiiied and applied across the resistor 28 to'l provide a squelch closing `potential for application to the grid 24 of the squelch tube23.

Although high pass filters and selective sys'- .l f tems to selecta noise band above'the .band offrel quencies for communication, have been previ,' ously described, it is also understood that the se@` lected band `of frequencies can be belowthe aui dio-transmission limits. A low pass filter to accomplish this purpose is illustrated diagrammatfore, with an increase in noise level in a particular locality, and hence an increase in the noise voltage across resistor 26, .there is a simultaneous increase of opposing voltage across the resistor 28 and the squelch is prevented from opening.

Although the operation of the present system to close the squelch after the carrier has gone scription, it is mentioned briefly that the squelch 23 is closed to prevent reproduction of noise at the speaker in the continuously energized radio receiver system, either by the application of a predominantly positive voltage to the grid 24 of the tube 23, or by the reduction of the negative voltage applied thereto. The presence of noise voltages will 'produce a negative vpotential across the resistor 26, but likewise will simultaneously produce an opposing-voltage across the resistor 28 adjusted so that the latter voltage is great enough to oppose the potential across the resistor 26.

It is therefore apparent, from the above description, that with the differential action of voltages across the resistors 26 and 28 when they are in opposing relation, and the voltage across the resistor 28 is predominant, the squelch is closed However, when the voltage across the resistor 26 increases as a result of the applicaticn of a carrier to the receiver system, or at least remains the same, the voltage across the resistor 28 is at that same time being reduced, until the .negative potential at the resistor 26 predominates and the squelch is opened. Stated another way, the voltage at the squelch tube 23 equals the differences between the voltages across the two resistors or resistances 26 and 26. When the difference represents a voltage of sufficient negative polarity, the tube is open or ineffective, but when the voltage has a positive polarity the squelch is closed and remains closed.

Fig. 2 illustrates diagrammatically a modified means for selectingthe desired noise band to produce a potential across resistor 28. The ele- `off the air can be understood from the above deically in Fig. 4. In addition to elements in the selective circuit of Fig. 4 corresponding to those in4 Fig. 1, there is also provided a tuning and filter circuit including resistance 6I condenser 62. and capacitances or condensers 6.3 and 64 together with the inductance 66.

While the squelch control system of the present invention has been described as used for squelching or muting the audio system in receiver systems where a maximum frequency of 3,000 cycles have been used in the audio system, it is likewise obvious that the operation of this squelch control system is not limited to audio-frequencies. The same principle, with a differential-action between v the voltages developed across different resistances may be applied to the control of a squelch, a relay, cr other device, where radio frequencies are used for the modulating signal. Since the noise spectrum in the presence ofa carrier in the output of an FM receiver is triangular in character, that is, the magnitude of the noise increases with increase in frequency, it is obvious that the system of this present invention may be used where the communication signal is a high frequency, and the noise used for developing the operating voltage for a squelch or other device, is a low frequency. A low passv filter may be used to select a noise band, and prevent the signal or communication from passing through a filter, while the operating circuit is designed to make luse of -tive portion of the complete system.

Although the preceding description has been concerned with the use of the differential action for the two voltages to operate a squelch tube, this differential action is equally applicable to the control, or operation of a relay instead of a tube to accomplish the control of the reproduction of the speaker. Such an application is illustrated in Fig. 7 shown in a manner similar to a diagram of Fig. 6. The elements or portions of the circuit corresponding to that of Figs. 1 and 8 have the same reference characters in Fig. 7. A relay control circuit is substituted for the squelch tube of Fig. 6, and this circuit includes D. C. amplifier tube 1| with a grid 12 connected to the resistor 28. A relay R includes an energizable coil 13 connected to the tube 1l, and a movable armature 14, as a switch arm for the contact 16 to open and close the circuit of the speaker S, or audio channel, in the same manner as the squelch tube of the circuit of Fig. 1. With the differential action of the voltages at the resistors 28 and 28, as previously described, when noise predominatea the positive potential on resistor 28 acting through the tube 1| energizes the relay and breaks the speaker circuit through the amature 14 and contact 16. Upon the introduction of a carrier at the input to the receiver, the noise voltage is reduced, -as previously described, the relay is deenergized, and the spring 11 pulls the amature into contact with 16 to close the speaker circuit. More particularly in the latter case, a signal applied to the input of the receiver causes an increase in voltage across resistance 28, while that across 28 decreases, re-` sulting in a substantial increase in voltage between grid 12 and ground. The resulting decrease in plate current in the tube 1| will cause coil 18 to be deenergized and the armature 14 to be released.

The squelch system of the present invention as above described is not limited to use in an FM receiver system but may also be applied to an amplitude modulated receiver. Such a system is illustrated in Fig. 8, in which all elements on the diagram corresponding to elements in Figs. 1, 2 and 3 with similar functions are numbered the same. In Fig. 8, the squelch opening voltage is produced across resistor 28 in a manner similar to that described in the description of the action of the FM receiver. In the present embodiment the rectiiied current produced at the amplitude detector or AVC rectifier 8| passes through resistance 28 to produce the desired controlling voltage. In order to provide a controlling voltage in opposition to that produced at resistance. 28, a portion of the radio frequency voltage available at the output of amplier I4, is coupled to the input of amplier 82, and the amplifier 82` output is coupled to the balanced detector 83. Balanced detector 88 is so designed that it is unresponsive to changes in amplitude of the signal input. The output of detector 88 will therefore provide random noise voltages similar to those available at the output of the FM detector i9 in Fig. 1. 'Ihe noise voltages coupled through condenser 81 and resistance 38 will be amplified by tube 28, selected by the band pass filter including the elements 48, 44, 42 and 4l, and rectified by rectiiler 2|, so that the rectified currents nowing through resistance 28 will produce a voltage across such resistance or resistor. This voltage appearing across resistor 28 is connected in series with the voltage appearing across resistance 28 in such a way that the differences between the voltages across resistors 28 and 26 will appear between ground and grid of squelch tube 28.

This system of Fig. 8 will now work in a manner similar to that described for the action of the system shown in Fig. l. 'I'he application of a carrier at the input to the receiver will increase the squelch opening voltage across resistor 28 and decrease the squelch opposing or a squelch closing voltage across resistor 28. This difierential change in voltages across resistors 26 and 28 will result in an increase in the negative potential between grid 24 and ground of tube 23, and that change in potential will be equal to the sum of the changes in potential produced across resistor 28 and the change in potential across resistor 26. The absolute value of voltage between grid 24 and ground will be equal to the difference between the voltage across resistor 28 and the voltage across resistor 26.

Fig. 9 is a modification of the circuit of Fig. 8, merely 'as to the addition of a limiter 84 after the amplifier 82 and ahead of the detector 83. The diagram of Fig. 9 is fragmentary but it is believed to be unnecessary to duplicate the similar portions of Fig. 8. The purpose of the limiter 84 is to reduce or eliminate amplitude variations of the signal fed to the detector 88. Where a balance R. F. and detector system is used the noise reduction produced by the carrier input to the receiver will follow without the use of a limiter, but such noise reduction may not take place in the event that all circuits are not balanced and the detector is not in a perfectly balanced condition. The limiter serves to remove amplitude variations and so make the noise reduction less dependent upon the balance of the circuits and therefore to make the system less critical.

The fundamental operation of the complete system illustrated fragmentarily in Fig. 9 is essentially the same as that disclosed for Fig. 1. The essential factor in the systems of Fig. 8 and Fig. 9 necessary to produce the squelch action described for Fig. 1 is the noise reduction at the output of detector 83 when a carrier is fed into the input of the receiver. In'Fig. 9, this noise reduction is achieved by'the use of limiter 84 which removes the hiss or noise voltages whenever the carrier signal input at antenna A is approximately twice as great as the noise peaks or voltages produced by thermal agitation, uctuation noise, or interference at theinput of amplifier I0. This same effect may be achieved by the use of a balanced detector associated with a properly balanced amplier. However, it may also be desirable to use both the limiter and the balanced detector system for best results.

' In the present invention, therefore, I provide a radio receiver circuit including means for developing a voltage for operating control means within the receiver circuit, and particularly for operating a squelch or muting means. The invention may be practiced in either a frequency modulated receiver or an amplitude modulated system. The preferred embodiment, particularly, in which a differential action between two voltages developed within the receiver circuit is utilized to in turn provide a single operating potential for operating a squelch, has proven especially effective for sensitive, positive operation under a wide range of conditions as to noise and carrier levels. Because it is equally effective at high as well as low noise levels the communication equipment utilizing my invention has a much more widespread and emcient use than similar equipment of the prior art. cause of its extreme sensitivity and its response to low carrier levels the service range for the equipment has been materially increased. In addition, under all of these circumstances the operator has the most pleasant as well as effective' receiving conditions for his equipment.

Although the invention has been described merely in its preferred embodiments, it is under- Furthermore, be-

stood thatit is not limited thereto, but is limited only by the scope of the appended claims.

I claim:

l. The method of operating a squelch circuit in a radio receiver system which includes in the total absence of a carrier from the system, selecting a band of noise frequencies generated in the system and being outside the frequency of the desired signal, rectifying the selected band of frequencies, developing a potential from the rectified current, applying said potential to the squelch circuit to operate the same, reducing said noise potential without tuning in the presence of a carrier in the system to a value such that a diiferent operation of said squelch may be accomplished. and applying said reduced potential to said squelch circuit.

2. In a radio receiver system having muting means therein, vthe means for controlling said muting means including a pair of resistances for developing a voltage across each electrically connected together in a manner to apply the voltages developed thereacross in opposed relation, electrical means connecting said resistances and said muting means to apply a voltage ldifference between said two voltages to said muting means, with one of said resistances having a voltage of a substantially predetermined magnitude developed thereacross, means in electrical connection with said one resistance and preceding said resistance in said system for selecting a'band of frequencies outside the frequencies normally reproduced by the receiver system said latter means including an. amplifier and filter circuit, and means intermediate said last mentioned means and said one resistance to rectify the selected voltages and apply a rectified 'current to said one resistance.

3. In a radio receiver system having a reproducer, the means for determining the operation of said reproducer including a first voltage developing means, a second voltage developing means electrically connected into the receiver system and with one another in a manner so that the developed voltages are normally in opposed relation, a squelch circuit in electrical connection with said reproducer and said two voltage developing means to be acted upon by any voltage difference between said two means, and said second voltage developing means including in electrical connection with one another, limiter means, detectar means, amplifier means, a resistance in electrical connection with said squelch circuit, and frequency selecting and voltage rectifying means intermediate said amplifier means and said resistance.

4. In a radio receiver having reproducing means and adapted to normally be maintained at a predetermined tuned position, the combination of a signal receiving and amplifying portion acting to generate noise voltages therein in the absence of -a carrier in the receiver, a balanced detector portion, and means electrically connecting said detector portion and said reproducing means for controlling the operation of said reproducing means, including a frequency selective portion in electrical connection with the balanced detector portion for selecting' a noise frequency different from the signal frequency for the receiver, a rectifier portion connected lto said frequency selective portion, and means in electrical connection with therectier portion for developing a voltage from the rectified current, and means connected with the voltage developing means and the reproducing means for amano determining the operation of said reproducing means.

5. In a radio receiving system which includes a signal responsive device, apparatus operative to transmit noise appearing in said system in the total absence of a received signal and also oper-I ative, without any tuning of the system, to reduce the noise transmitted thereby below a predetermined value when a desired signal is received, means for'selecting noise transmitted by said apparatus, and means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for preventing said device from responding to noise or signals appearing in the system-so long as the noise transmitted by said apparatus exceeds said predetermined value.

6. In a radio receiving system which includes a signal responsive device, apparatus operative to transmit noise appearing in said system in the total absence of a received signal and also operative, without any tuning of the system, to reduce the noise transmitted thereby below a predetermined value when a desired signal is received, means for selecting a portion of the noise transmitted by said apparatus, and means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for preventing said device from responding to noise or signals appearing in said system so long as the noise transmitted by said apparatus exceeds said predetermined value, said noise selecting means also being operative to prevent said lastnamed means from responding4 to a received desired signal.

7. In a radio receiving system which includes a signal responsive device, means responsive toa received desired signal or to noise appearing in said system for producing a i'lrst control voltage. apparatus operative to transmit noise appearing in said system in the total ab'sence of a received signal and also operative, without any tuning of the system, to reduce the noise transmitted thereby below a predetermined value when a desired signal is received, means for selecting noise transmitted by said apparatus, means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for producingl a second control voltage which is in opposition to said first control voltage and exceeds said first control voltage so long as the noise transmitted by said apparatus exceeds said predetermined value, and means controlled in accordance with the difference voltage between said first and second control voltages for preventing said device from responding to noise or signals appearing in said system `whensaid second lcontrol voltage exceeds said `first control voltage.

8. In a radio receiving system which includes a signal responsive device, means responsive to a received desired signal or to noise appearing in said system for producing a first control voltage, apparatus operative to transmit noise appearing in said system in the total absence of a received signal and also operative, withoutl any tuning .of the system, to reduce the noise transmitted thereby below a predetermined value when a desired signal is received, means for selecting noise transmitted bysaid apparatus, means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for producing ay second control voltage which is in opposite to said first control voltage and exceeds said first control voltage so long as the noise transmitted by said apparatus exceeds said predetermined value, said noise selecting means also being operative to prevent a received desired signal from substantially affecting the magnitude of said second control voltage, and means controlled in accordance with the diiference voltage between said ilrst and second control voltages for preventingsaid device from responding to noise or signals appearing in said system when said second control voltag exceeds said first control voltage.

9. In a frequency modulated radio receiving channel which includes a signal responsive device, amplitude limiting and frequency discriminating apparatus serially included in said channel and operative to transmit noise appearing in said channel in the total absence of a received signal, said apparatus also being operative, without any tuning of the channel, to reduce the noise transmitted thereby below a predetermined value when a desired signal is received, means for selecting noise transmitted by said apparatus, and means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for preventing said device from responding to noise or signals appearing in said channel so long as the noise transmitted by said apparatus exceeds said predetermined value.

10. Ina frequency modulated radio receiving channel which includes a signal responsive device, amplitude limiting and frequency discriminating apparatus serially included in said channel to limit the amplitude of a received signal modulated carrier and to detect the signal com- -ponents of the received signal carrier, said apparatus being operative to transmit noise appearing in said channel in the total absence of a received signal carrier and also being operative, without any tuning of the channel, to reduce the noise transmitted thereby below a predetermined value when a desired signal carrier is received, means for selecting noise transmitted by said apparatus, and means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for preventing said device from responding to noise or signals appearing in said channel so long as the noise transmitted by said apparatus exceeds said predetermined value, said noise selecting means also being operative to prevent said last-named means from responding to signal components detected by said apparatus during the reception of a desired signal.

11. In a frequency modulated radio receiving channel which includes a signal responsive device, means responsive to a received desired signal or to noise appearing in said system for producing a first control voltage, amplitude limiting and frequency discriminating apparatus serially included in said channel to limit the amplitude of a received signal carrier and to detect the signal components of the received signal carrier, said apparatus being operative to transmit noise appearing in the channel in the total absence oi' a received signal carrier and also being operative, without any tuning of the channel, to reduce the noise transmitted thereby below a predetermined value when a desired signal carrier is received, means for selecting noise transmitted by said apparatus, means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for producing a second control voltage which is in opposition to said first control voltage and exceeds said first control voltage so long as the noise transmitted by said apparatus exceeds said predetermined value, and means controlled by the difference voltage between said first and second control voltages for preventing said device from responding to noise or signals appearing in said channel when said second control voltage exceeds said rst control voltage.

12. In a frequency modulated radio receiving channel which includes a signal responsive device, means responsive to a received desired signal or to noise appearing in said system for producing a rst control voltage, amplitude limiting and frequency `discriminating apparatus serially included in said channel to limit the amplitude of a received signal carrier and to detect the signal components of the received signal carrier, said apparatus being operative to transmit noise appearing in the channel in the total absence of a received signal carrier and also being operative, without any tuning of the channel, to reduce the noise transmitted thereby below .a predetermined value when a desired signal carrier is received, means for selecting noise transmitted by said apparatus, means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for producing a second control voltage which is in opposition to said first control voltage and exceeds said rst control Voltage so long as the noise transmitted by said apparatus exceeds said predetermined value, said noise selecting means also being operative to prevent the detected signal components of a received signal carrier from substantially affecting the magnitude of said second control voltage, and means controlled by the difference voltage between said first and second control voltages for preventing said device from responding to noise or signals appearing in said channel when said second control voltage exceeds said rst control voltage.

13. In a radio receiving system which includes a signal responsive device, apparatus operative to transmit noise appearing in said system in the total absence of a received signal and also operative, without any tuning of the system, to reduce the noise transmitted thereby below a predetermined value when a desired signal is received, means for selecting noise transmitted by said apparatus, means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for preventing said device from responding to noise appearing in the system so long as the noise transmitted by said apparatus exceeds said predetermined value, and means for adjustably changing the predetermined level of noise at which said device is prevented by said last-named means from responding to noise appearing in the system.

14. In a radio receiving system which includes a signal responsive device, means for producing a first control voltage, apparatus operative to transmit noise appearing in said system in the total absence of a received signal and also operative, without any tuning of the system, to reduce the noise transmitted thereby below a predetermined value when a desired signal is received, means for selecting noise transmitted by said apparatus, means coupled to said noise selecting means and controlled by the noise selected by said noise selecting means for producing a second control voltage which is in opposition to said first control voltage and exceeds said first control voltage so long as the noise transmitted by said apparatus exceeds said predetermined value, means controlled in accordance with the difference voltage between said first and second control voltages forpreventing said device4 from responding to noise appearing in said system when said second control voltage exceeds said irst control voltage, and means for adjustably changing the relative magnitudes of the tlrst and second control voltages which are produced for any given level of noise transmission by said apparatus, thereby to change the noise level at which the difference voltage between said rst and second control voltages becomes effective to render said 6 signal device non-responsive to noise appearing in said system.

DANIEL E. `NOBLE. 

